The sequential appearance of B lymphocyte precursor cells has been defined by use of a short term induction assay which measures the conversion from one phenotype to another as cells pass from one differentiation compartment to the next. Our previous work has shown that four serologically defined surface markers, sIg, Ia, CR and PC.1 are inducible on four distinct B progenitor cell subpopulations, and from the phenotype analysis of these cells, a sequence of B cell differentiation has been proposed. Recent discoveries of the new alloantigenic systems Lyb2, Lyb3, Lyb4, Lyb5, ALA-1, and Ig5, and the novel immunoglobulin isotype sIgD, warrant an expanded analysis of the sequence of B cell differentiation by induction of phenotype conversion. We will determine the conditions of induction and the tissue sources of inducible precursor cells for each surface antigen, and analyze the surface phenotype of each inducible cell type by use of negative immunoselection techniques to find its particular location in the B lymphocyte lineage relative to other B cell subpopulations. We will clarify whether the induction of formerly unspecified immunoglobulin isotypes includes both sIgM and sIgD. If both are inducible we will determine their ontogenetic relationship. Furthermore, we will investigate the developmental association of early B cells containing cytoplasmic IgM with those expressing surface IgM.